Detection methods for particular biological molecules (bio-molecules) are manifold and several different approaches are presently available to the skilled person. In one method low concentrations of a bio-molecule (the target) in a liquid analyte mixture are detected by means of detection of Surface Plasmon Resonances (SPR).
Surface plasmon resonance (SPR) based biosensors detect changes in a sample by detecting changes in the index of refraction of the sample, and thus do not require any fluorescent or other labelling of the sample. Accordingly, they are known as label-free biosensors. A change in the sample causes a change in the index of refraction of the sample, which is also reflected in a frequency change, a wavelength change or other measures, which typically is measured to determine the change in the index of refraction of the sample and thereby a change in the sample.
The principle of surface plasmon resonance biosensors can be extended to a general change in surface polariton resonances. A polariton is an electromagnetic wave coupled to a polarization excitation is solids. When this coupled excitation is bound to the interface between two media it is called a surface polariton (SP).
In SP-based sensors, one known technique consists in exciting a so-called long-range surface plasmon polariton (LRSPP). An LRSPP consists of coupled surface plasma waves existing on opposite sides of a thin conducting film suspended between two dielectrics. For a system where the long-range surface polariton is coupled between opposite faces of a non-conducting material is often referred to as a long-range surface exciton polariton (LRSEP). The advantage of LRSPP-based and LRSEP-based detection is a decreased resonance width and thereby an increased sensitivity, as compared to standard SPR-based detection. In LRSPP-based and LRSEP-based sensors, the electromagnetic field is less confined to the surface of the sensing layer. Therefore these types of sensors are interesting in connection with detection of large molecules, such as viruses.
The typical SP-based sensor, including the LRSPP-based sensor, is based on having a thin conducting metallic film as the sensor layer in which the excitation occurs. In the published international patent application WO 01/88509 it is disclosed to use a metallic or semiconducting material for supporting surface plasmon polaritons, however it is also specified in the specification of the application, that materials with a small value of refractive index n (real part of the complex permittivity) and a large k (imaginary part of the complex permittivity) that will generate a surface wave. Such a material is generally considered as a material having metallic properties.
The inventors of the present invention have appreciated that an improved way of performing surface polariton measurements is of benefit, and have in consequence devised the present invention.